In one type of microfluidic system the conduit system is defined by micro-sized grooves or channels made in a first plate member and covered by a second flat plate member applied against the first plate member. A conventional material for the plate members is silicon. For the provision of adequate sealing between such silicon members in order to prevent fluid leakage between adjacent conduits or conduit portions, the plate members are bonded to each other, e.g. by glueing, anodic bonding or heating, to make the contact surfaces fuse together. Such a microfluidic silicon structure permits extremely small conduit dimensions, e.g. as low channel heights as 1 micrometer or even less.
For certain applications, however, it is required that the two plate members defining the microfluidic system are not permanently bonded together but may be repeatedly brought apart and put together, respectively. One such application is disclosed in, for example, WO 90/05295 relating to an optical biosensor system based upon surface plasmon resonance (SPR) and comprising a set of microfluidic flow cells for contacting the sample fluid with a sensing surface. Upwardly open flow-through cell channels are recessed into the surface of one plate member and closed by a second plate member supporting the sensing surface on the side thereof facing the first plate member. To provide for the necessary mutual sealing of the flow cells defined between the two plates when pressed together (conventional sealing means of the o-ring type or the like not being feasible in microscale connections) the upwardly open flow cell channels are formed as precision-cast cuts or slits extending through a top layer of an elastomeric material integral with the first plate member. Thereby efficient sealing is obtained when the sensing surface member is pressed against the elastomeric layer and a relatively large number of such dockings may be performed with retained sealing effect. With such an elastomeric layer channel heights as small as 50 micrometers may be provided, a practical lower limit being about 20 micrometers. Channels of still smaller heights may, however, not be made with adequate accuracy.
It has now been found that for obtaining optimum mass transfer conditions in, for example, the above mentioned biosensor application in order to obtain faster analyses and/or a higher sensitivity and/or a more reagent-saving system and/or more accurate kinetic measurements, it would be desired to use a maximum flow cell height of about 10 micrometers. While the desired low height channels, as mentioned above, can be performed in hard materials like silicon, there has so far been no satisfactory solution to the sealing problem associated with non-resilient materials.